A single permutation of the letters ABC is BAC.
Consider the following question...
You were over at my house yesterday doing math homework. You accidentally forgot your binder and you need it now. But since I don't skip class I just tell you the password to my house's codelock. When you arrive at my house, you realize you forgot the combination. You remember the numbers, 8, 2, 1, 5, 6, but not the order they are supposed to be in. You are tempted to just start guessing random combinations. But how long would it take?
The passwords to my house is a permutation (meaning that is has to be in a specific order) and not a combination (where it doesnt matter the order as long as it includes all the correct numbers). You have only tried 5 and realized it would most likely take a very long time.
To figure out the exact number of combonations you would use the Fundamental Counting Principle formula.So there are 120 ways to put the 5 numbers into my house's comboination lock.
I hope this example question helps you realize what permutations are all about. :)